Regulated synthesis of nitric oxide (NO) from arginine plays a key role in normal neural development, signalling, and response of the brain to trauma and infections. However, excessive NO production (as may result from trauma or inflammatory stress) can cause cell injury and/or death resulting in impaired brain function. Our data show that arginine synthesis is coordinately regulated with nitric oxide synthase (iNOS) induction in bacterial lipopolysaccharide-or S100B-treated astrocytes. Augmentation of argininosuccinate synthetase (AS), which catalyzes the rate-limiting step in arginine synthases, facilitates recycling of citrulline to support ongoing NO production under conditions of arginine depletion. We hypothesize that an arginine-nitric oxide cycle functions in neural tissue to insure that arginine availability does not limit NO synthesis. Several lines of evidence suggest that altered brain metabolism in response to these mediators may play a role in the development of mental retardation. Epidemiological studies have shown an association between intrauterine or perinatal infectious and development of mental retardation in some non-genetic causes of mental retardation. For patients with inborn efforts of metabolism, hyperammonemia during intercurrent infections is though to be causative in generating neural damage. S100B (which has been shown to induce neurotoxic levels of NO) maps to the Down syndrome region of chromosome 21. Its expression is elevated throughout development in Down Syndrome patients. In the proposed studies we will perturb brain cells with lipopolysaccharide (LPS, to mimic infection) or with S100B and we will determine the mechanisms which influence the capacity of brain cells to synthesize arginine under steady-state and stress conditions. We will evaluate the possible compartmentation of arginine and nitric oxide metabolism in cerebral cortex and cerebellum, two regions of the brain which differ in their content of nitrergic neurons. In addition, we will determine the effects of LPS-stimulated inflammatory stress on the flux of nitrogen metabolism through arginine versus glutamine synthesis. The molecular mechanism(s) by which LPS and S100B coordinately up-regulate argininosuccinate synthetase and inducible nitric oxide synthase in astrocytes will be determined using biochemical and molecular genetic techniques. We will examine AS enzyme kinetics, synthesis and gene expression. These studies will provide insight into alterations of brain nitrogen metabolism which occur in response to infection or trauma and will lay the groundwork for designing therapeutic strategies to limit their neurological consequences.